Iron pyrite, sometimes known as fool's gold, is one of the most abundant compounds found on the earth's surface. Pyrite is an iron (II) disulfide with an NaCl type cubic lattice structure and is weakly paramagnetic. Historically, the investigation of pyrite dates back to the mid 19th century with a focus of interest in biological applications. Despite its simple structure, pyrite has a low symmetry (space group Pa3), and therefore it is chiral making it biologically active. Generally speaking, iron based sulfides are an integral component of the earth's geological sulfur cycle. So, there are numerous iron sulfide compounds that form naturally. Several of the more common compounds include: marcasite, troilite, greigite, mackinawite, and smythite. Marcasite is the closest analog to pyrite, as it is also an iron (II) disulfide. The only structural difference is that it is orthorhombic and not cubic. This gives rise to several key differences in optical properties, which highlight the importance of phase purity when considering pyrite as a material for photovoltaics.
Pyrite is vastly different and in most ways superior in both bandgap energy and absorption coefficient α over most conventional semiconducting compounds considered for photovoltaics. Coupled with its environmental compatibility and earth abundance it's a natural candidate for further investigation. The bandgap of pyrite is at 0.95 eV. This is an indirect transition but there is a direct transition at ˜1.03 eV which becomes the energy gap of primary interest. The absorption coefficient α over the energy spectrum from wavelengths of λ=300 nm to λ=750 nm is a≈3.3×105 cm−1 which yields a 30 nm absorption length. By the Beer-Lambert Law, where I(λ)=Ioe−αt, one pass through a single absorption length will enable a material to absorb ˜63% of the incident photon energy from the sun. Assuming internal reflection and a back reflecting surface as is common in most photovoltaics, photons will have two passes through 30 nm giving an ˜87% theoretical absorption. In dramatic comparison, crystalline silicon requires a four order of magnitude larger film thickness (i.e. ˜300,000 nm) to achieve the same absorption. Pyrite is a good choice for further investigation, because its lower thickness for an absorption similar to that of silicon solves a material consumption issue by minimizing the input of material for TWh (terawatt-hour) scale production, and reduces bulk recombination due to shorter diffusion lengths. These two features however, are dependant on the ability to fabricate a 30 nm film.
In addition to the ideal physical and optical properties of pyrite as a candidate for photovoltaic material, it has the added benefit of being a sustainable choice. It is believed that pyrite has the ability to exceed annual global electricity production by several orders of magnitude using only a fraction of the material available. Furthermore, the mining of it is at such a low cost, it could become the least expensive semiconducting material for any application. Lastly, the material is non-toxic and naturally occurring, which delivers significant advantage over modern thin film photovoltaic materials, like cadmium telluride. The remaining challenge and opportunity for pyrite is throughput and rate of production. By virtue of being earth abundant, the real question is not in the production of raw materials but in the processing of that material.
Synthetic work on pyrite has been an active area of research for many years. Prior work in this field may be categorized into thin film growth and pyrite powder synthesis. Thin film work has ranged from: MOCVD, sputtering, electrodeposition, and sulfurization. Techniques exploring pyrite powder growth have included: hydrothermal, solvothermal, and inorganic colloidal approaches.
Perhaps the most significant and notable work on pyrite photovoltaics dates back to the early 1980's. Work led by Helmut Tributsch from the Hahn-Meitner-Instituit in Germany progressed toward working pyrite photovoltaic devices of 2.8% power conversion efficiency. They experimented with both a dye sensitized architecture and a homojunction where pyrite was doped n and p by varying concentrations of cobalt. Their best reported devices had an Jsc of 42 mA/cm2 and an Voc of 200 mV as taught by Ennaoui et al. in, Preparation of iron disulfide and its use for solar energy,” in Proc. of the First World Energy Congress, 1990. Although, they describe a high surface defect density as the root cause for lower than expected device performance, its perhaps more likely that material purity could be playing a role, especially given the low reported open circuit voltage values. Typically the techniques employed required a high temperature process step. Yet at elevated temperatures, segregation of iron and sulfur species is promoted, which could change the stoichiometry and phase of the material being deposited. This suggests purity has been a key hurdle in their pyrite work. Tributsch and his colleagues identified this as an issue and surmised that the quality of pyrite cannot be improved by high temperature treatments and therefore suggest passivation techniques to work with lower quality material in Alternatt et al., “Specifying targets of future research in photovoltaic devices containing pyrite (FeS2) by numerical modelling,” in Solar Energy Materials and Solar Cells, 2002, 71(2), 181-95.
Other groups went in a different direction in the 1980's and investigated solution growth techniques for pyrite powders as opposed to direct deposition of pyrite films. In these reactions, various salts bearing the cation Fe+2 or Fe+3 were reacted with polysulfide compounds, often in the presence of H2S gas. Reactions carried out for as many as 24 hours at 100° C. yielded favorable results in that pyrite was formed. Yet little characterization on the purity of these growths was provided. This may be explained by the primary objective of these studies, which was to establish new insights into the mechanisms by which pyrite forms in nature. So, the value or appropriateness of these methods for photovoltaics appears to be limited.
While much of this work from the 1980's has either persisted quietly or been discontinued altogether, there has been a resurgence in the field of synthetic approaches for pyrite powder formation. An important discovery made by Rickard and Luther was that crystal growth of pyrite type sulfides of 3d-transition metals heavier than Ni as well as tellurides of Fe and Co are almost impossible to synthesize by using ordinary methods. Pyrite materials with more electrons than FeS2 have an antibonding character; thereby making formation progressively more difficult and perhaps requiring higher pressures during formation techniques. This was a necessary observation during this earlier phase of work on traditional inorganic synthetic approaches to open the door for new high-pressure techniques, namely hydrothermal and solvothermal reactions.
Much of this emerging work has been performed in non-U.S. research tabs. There are a number of publications that discuss new hydrothermal and solvothermat pathways to cubic pyrite by use of a pressurized autoclave reactor. For example, see: Chen et al., Low-temperature hydrothermal synthesis of transition metal dichalcogenides, Chem. Mater. 2001, 13, 802-805; Chen et al., Single-source approach to cubic FeS2 crystallites and their optical and electrochemical properties, Inorg. Chem. 2005, 44, 951-954; Kar et al., Solvothermal synthesis of nanocrystalline FeS2 with different morphologies, Chemical Physics Letters, 2004, 398, 22-26; Kar et al., Wet chemical synthesis of iron pyrite and characterization by Mössbauer Spectrorscopy, Materials Letters, 2004, 38, 2886-2889; and Xuefeng et al., Solvothermal synthesis and morphological control of nanociystalline FeS2, Materials Letters, 2001, 48, 109-111. Yet, there are concerns with these studies involving both purity and morphology of the resulting product. A review of this work reveals a general inconsistency in reported XRD (x-ray diffraction) spectra. On close inspection, many of the XRD patterns presented are of either low quality or do not correspond to pure phase pyrite. This raises some doubts in the purity of the materials synthesized.